Genetic analyses of homeotic mutants in two model plants led to a genetic model of flower development( Carpenter and Coen, 1990 Schwarz-Sommer et al., 1990 Bowman et al., 1991). In these species, rapid progress has been made toward the understanding of the genetic regulation of floral organ specification. Genetic and molecular mechanisms of flower development have been intensively studied in two dicot species, Arabidopsis thaliana and Antirrhinum majus. Based on genetic and molecular results, we postulate a model of stamen and carpel specification in rice, with DLas a novel gene controlling carpel identity and acting mutually and antagonistically to the class B gene, SPW1. These incompletely differentiated organs are neither stamens nor carpels, but have partial floral identity. The flower of the double mutant, spw1 dl-sup, produces incompletely differentiated organs indefinitely after palea-like organs are produced in the position where lodicules are formed in the wild-type flower. All four dl alleles cause midrib-less drooping leaves. In the weakest mutant, dl-2, most of the flowers are normal. The intermediate allele dl-1 ( T65), results in an increase in the number of stamens and stigmas, and carpels occasionally show staminoid characteristics. In these strong mutants, many ectopic stamens are formed in the region where the gynoecium is produced in the wild-type flower and they are arranged in a non-whorled, alternate pattern. In contrast, two strong alleles of the dl locus, drooping leaf-superman1 ( dl-sup1)and drooping leaf-superman2 ( dl-sup2), cause the complete transformation of the gynoecium into stamens.
We show that SPW1 is a rice APETALA3 homolog, OsMADS16. Two spw1 alleles, spw1-1 and spw1-2, show the same floral phenotype and did not affect vegetative development. The homeotic mutation spw1 transforms stamens and lodicules into carpels and palea-like organs, respectively. We analyzed recessive mutants of two homeotic genes in rice, SUPERWOMAN1 ( SPW1) and DROOPING LEAF ( DL).
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